The central nervous system (CNS) modulates muscle force by two mechanisms: through recruitment of motor units and through variation in their discharge rate. For the effective control of force, however, provision must be made by the CNS to account for the heterogenous properties of motor units comprising a muscle. The order in which a motor unit is recruited, as well as whether its force output is routinely modulated by changes in discharge rate, appears directly related to its physiological properties with respect to other motor units within the same muscle. At a higher level of organization, different control strategies must be employed by the CNS to modulate force among muscles with diverse functions. Muscles with widely different functions often contain motor units with unique properties. This project is concerned with several key aspects of the input-output relations of motor pools. Electrophysiological experiments are designed: 1) to determine among muscles with diverse functions which mechanism, recruitment or rate modulation, is used to grade muscle force; 2) to determine the conditions under which different types of motor units are involved in rate modulation of their force output; 3) to determine the effect of different peripheral and descending synaptic inputs on the recruitment and rate modulation of different types of motor units; 4) to determine the incidence and relative threshold for bistable discharge behavior among different types of motor units; and 5) to determine when during the post- natal period the correlation between motor-unit endurance and recruitment threshold is established. These studies will focus on selected muscles of the cat distal forelimb and hindlimb. Motor units supplying distal forelimb muscles are of special interest because their mechanical properties and synaptic organization differ from motor units of other areas of spinal motor organization. In the past, the overwhelming majority of studies concerned with the segmental motor control system have focused on hindlimb muscles of the cat. A long-term objective of this project is to determine what organizational principles are common between the forelimb and other areas of motor organization, and what specializations are unique to the forelimb because of the motor tasks it must perform. This, in turn, will provide valuable background information on how the segmental motor control system may be affected under a variety of neurological and injury-related disorders.